Potent combination antiretroviral chemotherapy may allow clearance of cells acutely or productively infected with human immunodeficiency virus type 1 (HIV-1). However, a subpopulation of stably and nonproductively infected CD4+ cells containing integrated provirus and capable of producing virus upon stimulation has been identified in HIV+ individuals (Chun et al., 1995.; Chun et al., 1997). While these cells are rare, they may be long-lived, unrecognized by immune defenses, and unaffected by antiretroviral drugs. Presumably, restriction of transcription of the integrated viral genome plays a role in the quiescent state of these infected cells. Few host cellular factors have known that can inhibit transcription of the HIV promoter and establish or maintain this viral reservoir. We demonstrated that YY1, a widely distributed human transcription factor, inhibits HIV-1 long term terminal repeat (LTR) transcription and virus production (Margolis et al., 1994). Recently we found that YY1 restricts HIV transcription in the host cell in cooperation with a second transcription factor, LSF (Romerio, Gabriel, and Margolis, J. Virol., in press). LSF (also known as LBP-1c, UBP-1, or CP-2) can inhibit HIV transcription in vitro, but has not been shown to do so in vivo. We found that LSF participates with YY1 in the formation of a complex that recognizes the initiation region of the HIV-1 LTR, and that LSF synergizes with YY1 in the selective repression of LTR expression and viral replication. Thus cooperative regulation by two host transcription factors targeted to the LTR may play an important role in the establishment or maintenance of a population of cells stably but nonproductively infected with HIV-1. To gain insight into the regulation of HIV gene expression in this small but important viral reservoir, we will study the molecular mechanisms through which YY1 and LSF recognize and downregulate the HIV-1 promoter (Specific Aims 1-4). This understanding will allow the study of the role of these repressor factors in HIV replication through the use of proviral constructs unresponsive to YY1 and LSF, and modulation of YY1/LSF function (Specific Aim 5). Ultimately, our studies seek to develop novel therapies that manipulate HIV-1 expression in cells unaffected by current chemotherapy.